In the search for oil and gas deposits, seismic surveys are typically performed in order to locate likely prospects (or areas) having oil and gas. If an area looks promising, then a well may be drilled so as to penetrate down to the reservoir. When drilling the well, samples of the rocks and sediments brought up from the borehole are tested directly to identify oil or gas bearing zones. As the drill bit penetrates through a zone of interest, samples of the rock or sediment are brought to the surface for testing.
In particular, two parameters, total organic carbon content (or TOC) and vitrinite reflectance, are used in the petroleum industry as diagnostic tools for determining potential oil or gas bearing zones. If the rock samples contain a sufficient amount of TOC and are in the appropriate range of vitrinite reflectance values, then there is a strong potential that oil or gas have been produced in the zone of interest. TOC indicates the quantitative amount of organic material present in the sample. Samples with a sufficient amount of organic carbon are candidates as sources for oil or gas. Vitrinite is an organic material made up of plant debris. Vitrinite reflectance indicates the thermal maturity of the sample. Oil and gas are generated in a narrow range of thermal maturities which is estimated by the vitrinite reflectance of a sample.
Prior art methods for testing for TOC are imprecise, slow, expensive or are unable to quantify the amount of TOC. One prior art method which is fast but imprecise involves a geologist or other trained person visually examining and smelling the sample. The more organic material that is present, the darker the sample will appear. Another prior art method of testing for TOC involves substantial chemical analysis. The amount of all of the carbon in the sample is determined by combusting a portion of the sample in an oxygen stream. The total carbon includes inorganic or mineral carbon, as well as organic carbon. The amount of mineral carbon is determined by acid digestion of another portion of the sample. To determine the amount of organic carbon, the amount of mineral carbon is subtracted from the amount of total carbon. This method is slow and destroys the sample.
A prior art method that tests for vitrinite reflectance is undesirable because it is inherently subjective and imprecise. It involves grinding and digesting a sample in acid to concentrate organic material. The remains are then examined with a microscope that has been equipped with an oil immersion lens. A human operator visually identifies those pieces that are vitrinite and estimates their reflectance.